In oil or gas well drilling operations, drilling fluid (or “mud”) is continuously circulated through the inside of the drill pipe and out the drill bit then back up to the surface. Drilling mud is typically made up of clays, chemical additives and an oil or water base. This fluid has several purposes: 1) controlling formation pressure; 2) cleaning the well bore of formation debris; 3) lubricating, cooling, and cleaning the drill bit and drill string; 4) stabilizing the well bore; and 5) limiting the loss of drilling mud to the subsurface formation.
In cleaning the well bore, the circulating drilling mud removes the drill cuttings as well as formation fluid trapped in the pore space or fractures of the rock. During the drilling operation, the entrapped formation fluid and gases in the drilling mud are monitored in real-time at the surface. The recording of the measurements is called mud logging. Mud logging measurements can include temperature, pH, drill rate, chlorides, total hydrocarbon content, and the concentration of specific formation gas components. These logs are important as they enable the drilling operator to ascertain the presence of oil or gas in the formation being drilled. Significant measured gas increases in the drilling mud during drilling indicate oil or gas bearing zones in the formation and are known as “shows”.
To measure the amount of formation gas entrapped in the drilling mud and determine the concentration in the formation fluid, several techniques have been used. A small amount of the drilling mud can be pumped through a mechanical agitation device known as a gas trap which is located at the surface. The purpose of the gas trap is to extract the gases from the drilling mud for measurement and analysis. Separation and quantification of the gas components, light hydrocarbon gases, is typically carried out by means of in-line gas-chromatography or gas-chromatography mass-spectrometry analysis. Gas trap sampling and analysis can be monitored continuously in real-time as part of typical mud logging activities, providing the drilling operator with real-time concentrations of the gas components per linear foot drilled for the entire depth of the well. The ability to distinguish formation fluid types, especially their gas to oil ratios (GOR), from analysis of the light gases is a highly desirable goal since time and resources spent on formation testing can be minimized. This data is essential for the economics and feasibility of any hydrocarbon reservoir.
Unfortunately, there are numerous problems associated with surface gas trap measurements. The relative concentrations of the different gas components extracted from the drilling mud and collected in the head space of the gas trap are not representative of the actual gas concentrations evolving from the drilling fluid. As a result, measured gas trap values are not representative of the gas composition of the drilling fluid or the formation fluid at depth. Additionally, uncorrected gas trap values can lead to widely divergent predictions of total fluid properties, including GOR determinations.